1. Field of the Invention
This invention resides in asphalt technology and asphalt formulations, and also in waste management, particularly in connection with recycled tire rubber.
2. Description of the Prior Art
The question of how to dispose of used or scrap automobile tires has been a matter of public concern since the mid-1970's. Over 250 million scrap tires are generated each year in the United States alone, and comparable problems exist in nearly every country.
Reclamation of the rubber from used tires is extremely difficult and costly. Reclamation requires that the vulcanized rubber in the tires be de-vulcanized by breaking carbon-to-carbon and carbon-to-sulfur bonds without destroying the polymer backbone chains. The rubber must also be separated from the resins, oils, carbon black and mineral fillers that are also part of the tire composition, and this must be done after the tires have been chopped and granulated and the steel and fibers from the belted layers removed. Most prior art processes for tire rubber reclamation involve mixing the granulated scrap tires with solvents, treatment chemicals, thermoset plastics, or mineral fillers and heating the resulting mixture to temperatures ranging from 37° C. (100° F.) to 400° C. (752° F.) under a pressure of 1 to 2.5 atmospheres, resulting in a plastic-like end product that can easily be processed, combined with natural or synthetic rubber virgin polymers if desired, and re-vulcanized. Patents disclosing these processes include Le Beau U.S. Pat. No. 2,783,213, issued Feb. 26, 1957, which discloses a process involving the addition of a mineral filler; Yankner et al. U.S. Pat. No. 4,157,320, issued Jun. 5, 1979, disclosing the use of a rubber processing aid made from petroleum absorbed on attapulgite clay, kaolin clay, or synthetic calcium silicate clay; Crivelli U.S. Pat. No. 5,258,222, issued Nov. 2, 1993, disclosing the use of siliceous crystalline grains with a vulcanizable polymer; and Chen U.S. Pat. No. 5,286,374, issued Feb. 15, 1994, disclosing the catalytic cracking of tire rubber at 240° C. (446° F.) to 400° C. (752° F.) under pressure in the presence of a mica catalyst such as muscovite, sericite, or bioltite, each of which is a form of potassium aluminum fluorosilicate.
The recycling of tire rubber has generated more interest than reclamation. Recycling involves granulation of the tires and removal of the steel and fiber so that the rubber can be used for alternative processes. Among the most common uses of recycled tire rubber is the incorporation of the rubber into bituminous materials, primarily petroleum asphalt. Early studies on the incorporation of tire rubber into asphalt were made by Charles H. McDonald as disclosed in his U.S. Pat. No. 3,891,585, issued Jul. 5, 1973, followed by his later U.S. Pat. No. 4,018,730, issued Apr. 19, 1977, U.S. Pat. No. 4,021,393, issued May 3, 1977, U.S. Pat. No. 4,069,182, issued Jan. 17, 1978, and U.S. Pat. No. 4,085,078, issued Apr. 18, 1978. With co-inventor Winters, in U.S. Pat. No. 3,919,148, Nov. 11, 1975, McDonald discloses the use of recycled tire rubber in combination with an elastomeric paving material to form a paving asphalt. Nielsen and Bagley, in U.S. Pat. No. 4,068,023, Jan. 10, 1978, disclose the addition of reclaimed rubber to molten paving asphalts with the further inclusion of low viscosity aromatic oils. Huff, in U.S. Pat. No. 4,166,049, Aug. 28, 1979, discloses a rubberized asphalt prepared from asphalt, oil, scrap rubber, and devulcanized scrap rubber. Yan, in U.S. Pat. No. 4,139,397, Feb. 13, 1979, U.S. Pat. No. 4,211,576, Jul. 8, 1980, and U.S. Pat. No. 4,278,469, Jul. 14, 1981, discloses paving asphalt compositions prepared by heating a mixture of coal tar and fluidized catalytic cracking syntower bottoms to achieve a product with asphalt properties. Gaus and Klabunde, in U.S. Pat. No. 4,310,446, Jan. 12, 1982, disclose a joint sealant composition that is compatible with asphalt and that contains an aromatic petroleum tar, an oil-soluble ground rubber, an inorganic filler, and polyvinylchloride. Von der Wettem and Albrecht, in U.S. Pat. No. 4,381,357, Apr. 26, 1983, disclose a road covering consisting of broken stone as mineral fillers, reclaimed rubber, and bitumen. John Eric Partanen, in U.S. Pat. No. 4,437,896, Mar. 20, 1984, discloses synthetic asphalt mixtures containing tall oil pitch, gilsonite, asphalt, aggregate, and recycled tire rubber. Davis, in U.S. Pat. No. 4,485,201, Nov. 27, 1984, discloses mixtures of asphalt, SBS polymer, and granulated tire rubber. Schmanski, in U.S. Pat. No. 5,290,833, Mar. 1, 1994, discloses coated ground tire rubber particles combined with asphalt, sand and gravel. Rouse discloses petroleum asphalt combined with minus-50 mesh recycled tire rubber particles in U.S. Pat. No. 5,334,641, Aug. 2, 1994, and with minus-80 mesh recycled tire rubber particles in U.S. Pat. No. 5,525,653, Jun. 11, 1996. Flanigan discloses the combination of distillation tower bottoms (asphalt) and tire rubber mixtures that are air blown at 6 to 15 pounds per square inch at temperatures of 176° C. (350° F.) to 260° C. (500° F.) in U.S. Pat. No. 5,397,818, Mar. 14, 1995, and various other methods of incorporating tire rubber into asphalt in U.S. Pat. No. 5,492,561, Feb. 20, 1996, and U.S. Pat. No. 5,583,168, Dec. 10, 1996. Truax discloses a mixture of foamed asphalt and tire rubber in U.S. Pat. No. 5,486,554, Jan. 23, 1996. Memon, in U.S. Pat. No. 5,486,554, Jan. 6, 1998, discloses the combination of asphalt and crumb rubber mixed with peroxide plus a glycidyl-containing monomer as an additive. Labib et al., in U.S. Pat. No. 6,478,951, Nov. 12, 2002, disclose a crumb rubber-modified asphalt in which the crumb rubber is coated with a compatibilizer containing a glycidyl group to more fully interact with the asphalt. John Eric Partanen et al., in U.S. Pat. No. 7,025,822, Apr. 11, 2006, disclose asphalt mastics containing refinery solids and including granulated recycled tire rubber. A U.S. patent application Ser. No. 11/744,399, filed on May 4, 2007 by John Eric Partanen et al. and pending as of the filing date of the present application, discloses combinations of asphalt, granulated recycled tire rubber, and spent pulverized recycled activated carbon. Other patent documents of John Eric Partanen relating to asphalt compositions that incorporate recycled tire rubber are published patent applications US 2005/0011407 A1 (Jan. 20, 2005), US 2005/0027046 A1 (Feb. 3, 2005), US2006/0130704 A1 (Jun. 22, 2006), and US 2007/0049664 A1 (Mar. 1, 2007). The compositions in these latter Partanen documents are asphalt/water emulsions that are further water-dilutable and designed for use as a slurry seal to be applied as a thin coating on streets and highways.
An alternative to reclamation and recycling is pyrolysis. Pyrolysis is performed at temperatures in excess of 260° C. (500° F.) in the absence of oxygen and produces a product mixture containing about 6% gaseous products, 55% oily products, 9% steel, 5% fiber, 19-20% carbon black, 2-3% mineral fillers, 1-2% sulfur, and 3-4% destroyed polymer, all by weight. The oily products can be burned to generate the heat required for the pyrolysis, and some of the solid products can be recovered and reused, although most of the solid products are typically disposed of as “flyash.” Pyrolysis generates large amounts of heat which can be utilized in conjunction with a cogeneration steam plant and converted to electrical energy. Air pollution control restrictions have severely limited this option, however.
A process to co-recycle large pieces of shredded rubber tires with residua, 600 w cylinder oil, waste motor oil, trim gas oil, vacuum heavy bottoms, decanted oil, and combinations thereof is disclosed by Cha in U.S. Pat. No. 5,735,948, Apr. 7, 1998. In this process, the recycled material is combined with crushed mineral fillers that are either calcium carbonate, calcium hydroxide, calcium oxide, magnesium carbonate, magnesium hydroxide, dolomite, sodium carbonate, sodium hydroxide, potassium carbonate, potassium hydroxide, iron oxide, bauxite or red mud from aluminum smelting, and the mixture is heated in a retort at 468° C. (875° F.) to remove water and light hydrocarbons. The process equipment includes a tire feed hopper, an oil feed system, variable-speed screw reactors with non-symmetrical tubes that are electrically heated, liquid receiver vessels, solid product receiver vessels, and glass and steel receiver vessels. A horizontal or vertical feed screw conveys the tires and oils to a first stage, primary screw reactor where the additives are injected and mixed and the tires are digested with the oils at 315° C. (600° F.) to 399° C. (750° F.). The tires are shredded in this stage and the separation of glass and steel from the rubber and other tire components is begun. Retorting continues in a second stage reactor, at process temperatures of 427° C. (800° F.) to 468° C. (875° F.). Volatile and light hydrocarbon components such as gases and liquids are collected and condensed, then stored or used as fuels for the process. A second horizontal reactor receives the carbonaceous residue from the first stage reactor and lowers the remaining oil content to below 1% at temperatures of 427° C. (800° F.) to 482° C. (900° F.). A claimed benefit to including the crushed mineral fillers in the reaction mixture is the elimination of hydrogen sulfide gas and the adsorbance of chlorinated compounds from the light oils collected. The use of carbonaceous residue rather than crushed limestone is claimed to: (1) improve the rheological properties of the asphalt; (2) improve binder aging; (3) increase the embrittlement of the asphalt binder; and (4) improve the force ductility at 4° C. (39.2° F.). When mixed with aggregates to produce hot mixes, binders containing the carbonaceous residue are claimed to: (1) increase the Marshall stability and lower the Marshall flow values, (2) improve the resilient modulus which indicated that these mixtures have higher load carrying capabilities with less tendency to rut; (3) show less tendency to rut in accelerated wheel rut testing with the Georgia loaded wheel tester; and (4) allow the mixtures to pass severe freeze-thaw moisture sensitivity testing with no failure for over 50 cycles, when most pavement mixtures without the carbonaceous additive failed in 8 cycles or less. The carbonaceous residue was also found to be of value as a Portland Cement additive after treatment with a surfactant.
A typical composition of granulated recycled tire rubber is as follows (all percents by weight):
63-64%heavy oil and hydrocarbon resins6-7%light oil21-22%carbon black2-3%inorganic mineral fillers1-2%sulfur3-4%polymers
When granulated tire rubber is added directly to hot asphalt, the rubber begins to melt or decompose. In the decomposition, the polymer dissociates from the sulfur and begins to break apart into light hydrocarbon components, including light oils that are volatile and escape in gaseous form from the mixture over time. At temperatures approaching 200° C. (398° F.), the rate of escape increases considerably and these light components leave the mixture entirely. While the passage of these components is in progress, most asphalt-rubber mastics of the prior art undergo a transitional stage in which their properties are changing due to the gradual loss of these components. To take advantage of, or to fully accommodate, these transitional properties, the mastics must be used within two hours of their preparation. The two-hour window is less critical in mastics prepared by the procedures disclosed in the Flanigan patents above, however, in which the mixture of tire rubber granules and asphalt is heated to 260° C. (500° F.) using jet spray nozzles in a circulating reactor. This causes rapid volatilization of the light components but involves expensive equipment that requires substantial upkeep. In addition, without special treatments such as the coating of the rubber granules, the granules tend to separate from the asphalt, resulting in a lack of homogeneity.